1. Field of the Invention
The present invention relates to an optical fiber suitable for an optical transmission line in a long-haul optical communication system.
2. Related Background Art
As optical transmission lines for transmitting light signals in an optical communication system, several types of optical fibers have been used or studied. For example, as the first prior art, a silica glass based fiber having a core region doped with germanium (to be referred to as a Ge-doped core optical fiber hereinafter) has been used. As the second prior art, a silica glass based fiber having a core region doped with no germanium and a cladding region formed around the core region and doped with fluorine (to be referred to as a pure silica core optical fiber hereinafter) has been used. As the third prior art, a silica glass based optical fiber is disclosed in Tanaka, et al., xe2x80x9cHigh Silica Core Single-Mode Fibers for 1.55 xcexcm Transmissionxe2x80x9d, Fujikura Technical Review, 1990, in which the core region is doped with chlorine to reduce residual stress (to be referred to as a chlorine-doped core optical fiber hereinafter) so as to effectively suppress deformation of the refractive index profile of the optical fiber obtained after drawing.
The inventors have found the following problems upon examining the above prior arts. An optical fiber for communication is required to have a small transmission loss. In consideration of the use of an optical fiber for a cable, it is also required that the bending loss be small. In order to reduce bending loss, the relative refractive index difference of a core region with respect to the refractive index of a cladding region must be increased to increase light confinement efficiency.
In the above Ge-doped core optical fiber as the first prior art, a reduction in bending loss can be attained by increasing the contents of germanium in the core region and increasing the relative refractive index difference between the core region and the cladding region. If, however, a large amount of germanium is added in the core region on which the optical power of incident light concentrates, the transmission loss increases because the Rayleigh scattering coefficient caused by the germanium is larger than that caused by pure silica. For this reason, in the Ge-doped core optical fiber as the first prior art, it is difficult to effectively reduce both the transmission loss and the bending loss.
In the pure silica core optical fiber as the second prior art, a reduction in bending loss can be attained by increasing the contents of fluorine in the cladding region and increasing the relative refractive index difference between the core region composed of pure silica and the cladding region. In the second prior art, however, even if the contents of fluorine in the cladding region increases, the transmission loss due to Rayleigh scattering is small because the core region is composed of pure silica. However, it is difficult from a production viewpoint to add a large amount of fluorine in the cladding region. In addition, since the differences in physical property value (e.g., viscosity upon heating) between the core region and the cladding region increase, the transmission loss due to structural mismatching at the interface between the core region and the cladding region increases. In the pure silica core optical fiber as well, therefore, it is difficult to effectively reduce both the transmission loss and the bending loss.
In the chlorine-doped core optical fiber as the third prior art, according to Tanaka, in laying the a submarine cable, the level of bending loss upon bending with a diameter of 20 mm needs to be 3 to 1 dB/m or less. Such a description, however, states only a required value of bending loss. However, there is no description in this reference about how to realize an optical fiber that can satisfy this required value of bending loss. In addition, there is no description about a specific level to which bending loss can be actually reduced.
The present invention has been made to solve the above problems, and has as its object to provided an optical fiber having a structure that effectively reduces both transmission loss and bending loss, and a method of manufacturing the optical fiber.
An optical fiber according to the present invention contains silica as a main component and includes a core region containing a predetermined amount of chlorine and a cladding region which is provided on the periphery of the core region and which contains a predetermined amount of fluorine. A characteristic feature of the optical fiber, in particular, is that the peak value of the relative refractive index difference of the core region with respect to the refractive index of pure silica is 0.05% or more.
In the optical fiber according to the present invention, since the Rayleigh scattering coefficient caused by chlorine added in the core region is small, the transmission loss due to Rayleigh scattering is small. In addition, since chlorine is added as a dopant in the core region, the differences in physical property value between the core region and the cladding region decrease, and the transmission loss due to structure mismatching at the interface between the core region and the cladding region decreases. Furthermore, since the peak value of the relative refractive index difference of the core region with respect to the refractive index of pure silica is 0.05% or more, the bending loss is sufficiently reduced.
The concentration of chlorine added in the core region preferably increases from a peripheral portion of the core region toward its center. By setting the concentration of chlorine in a peripheral portion of the core region to be lower than that in the center of the core region, the differences in physical property value between the core region and the cladding region further decrease. This further decrease the transmission loss due to structure mismatching at the interface between the core region and the cladding region.
The chlorine added in the core region diffuses into the cladding region in the process of manufacturing the optical fiber according to the present invention, and hence chlorine is contained in at least part of the cladding region.
In the optical fiber according to the present invention, an increase in transmission loss due to an OH-radical at a wavelength of 1.38 xcexcm is 0.5 dB/km or less. Such a reduction in transmission loss can be attained by sufficiently performing dehydration using a halogen gas in the process of manufacturing the optical fiber. Chlorine to be contained in the core region is also introduced in this dehydration step. In the case shown in FIG. 8 in the reference by Tanaka, an increase in transmission loss due to an OH-radical at a wavelength of 1.38 xcexcm exceeds 0.6 dB/km. That is, dehydration is not performed or not sufficiently performed.
The optical fiber according to the present invention has a zero dispersion wavelength at 1.34 xcexcm or more. By setting a zero dispersion wavelength to 1.34 xcexcm or more, chromatic dispersion at a wavelength of 1.55 xcexcm is reduced. This eliminates the necessity of dispersion compensation or allows optical transmission at a wavelength of 1.55 xcexcm with a small amount of dispersion compensation. By setting a zero dispersion wavelength in this manner, the transmission loss due to an OH-radical at a wavelength of 1.38 xcexcm can be suppressed to 0.5 dB/km or less. This allows the use of a 1.38 xcexcm band as a signal wavelength band.
In order to shift the zero dispersion wavelength to a long wavelength side while the relative refractive index difference between the core region and the cladding region is maintained constant, the diameter of the core region must be decreased. If, however, the diameter of the core region decreases, the bending loss increases. The bending loss can effectively be reduced by increasing the relative refractive index difference between the core region and the cladding region. However, as the relative refractive index difference is increased by increasing the contents of germanium in the core region, the transmission loss increases. In consideration of these points, according to this optical fiber, a sufficient relative refractive index difference is obtained by adding fluorine in the cladding region without containing germanium in the core region.
The optical fiber according to the present invention has an effective area of 100 xcexcm2  or more with respect to a wavelength band in use. If the effective area is 100 xcexcm2 or more, the non-linearity of the optical fiber is reduced. This makes it possible to effectively use this optical fiber in a case wherein a high-power optical input can be obtained as in WDM (Wavelength Division Multiplexing) transmission using an optical fiber amplifier.
Note that a depressed cladding structure is preferably used to effectively suppress an increase in bending loss and obtain a very large effective area. In the optical fiber according to the present invention, therefore, the cladding region may have a structure including an inner cladding provided on the periphery of the core region and having an average refractive index lower than that of the core region, and an outer cladding provided on the periphery of the inner cladding and having an average refractive index lower than that of the core region but higher than that of the inner cladding.
An optical fiber manufacturing method according to the present invention includes the first deposition step of obtaining a porous glass body serving as the core region, the dehydration step, and the first sintering step of making the porous glass body, obtained in the first deposition step, into a transparent glass body.
The above porous glass body is obtained by a vapor phase deposition method such as a VAD (Vapor phase Axial Deposition) method or OVD (Outside Vapor phase Deposition) method. The dehydration step is performed by heating the porous glass body obtained in the first deposition step in an atmosphere containing a chlorine element. At least the concentration of chlorine in the atmosphere must be adjusted such that the peak value of the relative refractive index difference of the core region with respect to pure silica in the obtained optical fiber becomes 0.05% or more.
This manufacturing method further includes the second deposition step of sequentially depositing one or more porous glass layers serving as a cladding region on the transparent glass body obtained in the first sintering step, and the second sintering step of making one or more porous glass layers, obtained in the second deposition step, into one or more transparent glass layers. Note that dehydration may be performed by heating the glass body in an atmosphere containing chlorine between the second deposition step and the second sintering step.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.
FIG. 1A is a view showing a cross-sectional structure of each of the first to fourth embodiments of the optical fiber according to the present invention, and FIG. 1B is view showing the refractive index profile of the optical fiber according to each of the first and fourth embodiments;
FIG. 2 is a view showing the refractive index profile of a pure silica core optical fiber as a comparative example;
FIG. 3 is a graph showing the result obtained by performing a simulation concerning the relationship between the mode field diameter and bending loss of the optical fiber according to each of the first to fourth embodiments and the pure silica core optical fiber as a comparative example (the relative refractive index difference between the core region and the cladding region is constant);
FIG. 4A is a view showing a cross-sectional structure of the fifth embodiment of the optical fiber according to the present invention, and FIG. 4B is a view showing the refractive index profile of the optical fiber according to the fifth embodiment;
FIGS. 5A to 5C are views for respectively explaining the first deposition step (FIG. 5A), the dehydration step (FIG. 5B), and the first sintering step (FIG. 5C) in the process of manufacturing the optical fiber according to the present invention;
FIGS. 6A and 6B are views for respectively explaining the second deposition step (FIG. 6A) and the dehydration step (FIG. 6B) in the process, of manufacturing the optical fiber according to the present invention; and
FIGS. 7A and 7B are views for respectively explaining the second sintering step (FIG. 7A) and the drawing step (FIG. 7B) in the process of manufacturing the optical fiber according to the present invention.